This invention relates generally to antennas and, more particularly, to antennas used in high performance military aircraft. There is a requirement for such aircraft to sense incident electromagnetic energy instantaneously over a broad frequency band, for example from 2 to 20 GHz (gigahertz). Moreover, the polarization of the incident radiation may be linear horizontal polarization, linear vertical polarization, right-hand circular polarization (RCP), or left-hand circular polarization (LCP).
A right-hand circularly polarized sensor antenna will detect linearly polarized or right-hand elliptical or circular polarized radiation, but will not detect electromagnetic radiation of the left-hand polarization sense. Therefore, a system equipped with conventional single-sense antennas, such as broadband spiral antennas, will be unable to detect electromagnetic radiation of the opposite polarization sense.
One solution to this difficulty is to provide both left-hand and right-hand spiral antennas. However, space for multiple antennas may not be available in some aircraft.
Sensor antennas in some aircraft are used to provide angle-of-arrival signals, i.e. to provide information concerning the angular direction of a signal source. The accuracy of angle-of-arrival systems can be improved if the sensor antenna can operate in higher order modes. In particular, a multi-mode antenna offers the advantage of increased gain at small elevation angles above the plane containing the antenna aperture. Military aircraft antennas are often installed in locations that compromise antenna performance, specifically the radiation pattern and gain. For example, antennas may be installed on the side or top of the fuselage and will have difficulty "seeing" in a forward or backward direction relative to the aircraft's direction of motion.
It will be appreciated that there is a significant need for a single antenna that overcomes all of the shortcomings mentioned above. As will become apparent from the following summary, the present invention satisfies this need.